So, you're telling me that if someone uses ext3 they will get a default
file system that's read/write, but if they choose any other available
file system it will be read only by default even though xen-tools.conf
doesn't list ro only as an option for mounting them?

No, I was specifically refuting your suggestion that by default
installations will fail.
The expectation is :
a. default filesystem ext3 - it will work.
b. reiser filesystem - it will work.
That it hasn't worked for you in this case is either a bug, or
error. I cannot tell which, though I'd like to lean towards a bug
that has not yet been reported by any users.
Steve